What Causes Missed Period And Positive UPT Test While On Depo Shot?

Im curious to know what is going on. I been on the depo shot since August 2014 I was due for my 3rd shot Feb 2015 bt missed my appt because we were snowed in bt lately my breast been tender and im not bleeding anymore I never got the shot and also to mention my tubes are tired bt I took a pregnancy test one line was faint...what do you think this could be??